1. Contaminant Metals in Catalytic Cracking
Hydrocarbon feedstocks containing higher molecular weight hydrocarbons can be converted into lighter weight products, such as gasoline, by the process of catalytic cracking. This process is adversely affected if certain metals, such as platinum, palladium, chromium, nickel, copper, vanadium, and iron are present. These metals are themselves hydrogenation-dehydrogenation catalysts and cause the increased formation of coke and hydrogen gas, thereby decreasing the yield of the desired gasoline. In addition, these metals affect both the activity and selectivity of the cracking catalyst.
Unfortunately, nickel, copper, vanadium, and iron are often present as contaminants in the hydrocarbon feedstocks which are catalytically cracked. For example, the metals level in the gas oils which traditionally have been the catalytic cracking feedstock is generally about 0.25 ppm Nickel Equivalent. The term "ppm Nickel Equivalent" is defined here as
ppm Nickel Equivalent=ppm nickel+ppm copper+(0.2)(ppm vanadium)+(0.1)(ppm iron)
Since the individual metals levels are weighted, this term takes into account that, if present at equal levels, the adverse effects of nickel and copper are substantially greater than those of vanadium, whose effects are, in turn, greater than those of iron.
Traditionally, the problem of contaminant metal deposition has not been serious because it is common practice to continually withdraw a portion of the catalyst in the unit, discard it, and replace it with fresh catalyst. While this withdrawal is primarily done to maintain catalyst activity (which decreases with time), it also has the effect of controlling the metals level on the catalyst at a level where the adverse effects are minimal.
As an example, assume that: (1) a catalytic cracking unit processes 50,000 bbls/day of gas oil, (2) the gas oil density is 390 lbs/bbl, (3) the gas oil has a metals content of 0.25 ppm Nickel Equivalent, (4) the fresh catalyst has a metals level of 25 ppm Nickel Equivalent, and (5) the catalyst inventory in the unit is 300 tons. Then, if the catalyst withdrawal rate is 1.5 percent, or 4.5 tons/day, the average metals level on the catalyst in the unit is about 600 ppm Nickel Equivalent, a level at which the adverse effects are minimal.
Recently, the problem of contaminant metal deposition has become more serious because the metals level in catalytic cracking feedstocks is rising. There are two major reasons for this rise. First of all, refiners have begun using more of the lower quality crude oils which contain higher levels of contaminant metals. And when the metals level in the crude is higher, the metals level in the gas oil fraction is also higher. Secondly, and more importantly, there now exists a great economic incentive to catalytically crack residual oils rather than to sell them for use as fuels. It is in the residual oil fraction that the contaminant metals in the crude are most concentrated.
As a result of these recent changes, the contaminant metals level in catalytic cracking feedstocks often greatly exceeds the traditional level of 0.25 ppm Nickel Equivalent. For example, gas oil fractions from lower quality crudes can exceed 1.0 ppm Nickel Equivalent and when blends of gas oil and residual oil are used, the metals level can reach 40.0 ppm Nickel Equivalent.
At these higher metals levels, catalyst withdrawal alone is no longer adequate to control the adverse effects of contaminant metal deposition. For instance, in the above example of a 50,000 bbl/day unit, an increase in the metals level of the feedstock from 0.25 to 1.0 ppm Nickel Equivalent has a tremendous effect. If the metals level on the catalyst is to be maintained at 600 ppm Nickel Equivalent, the withdrawal-replacement rate must increase fourfold to 18 tons/day. The cost of the catalyst itself and of materials handling prohibit significantly increasing the rate beyond the rate necessary to maintain catalyst activity. On the other hand, if the catalyst withdrawal-replacement rate is maintained at 4.5 tons/day, the metals level on the catalyst in the unit jumps to about 2400 ppm Nickel Equivalent, a level at which the adverse effects are intolerable.
Several approaches have been developed to supplement catalyst withdrawal-replacement when the metals level in the feedstock rises to about 1.0 ppm Nickel Equivalent. One approach is to use a separate metals-removing step before the hydrocarbon feedstock is catalytically cracked. This approach suffers from the disadvantages of being very costly to operate and of requiring a large amount of new equipment to implement. A second approach is to remove the metals from the cracking catalyst after they have been deposited and then reuse the catalyst. This approach is also costly to operate and requires new equipment.
A third approach, passivation, is to chemically treat the catalyst so as to reduce the tendency of deposited metals to catalyze the formation of coke and hydrogen gas. This approach has heretofore presented difficulties because many chemicals which are effective passivating agents are also highly toxic and/or very expensive. Examples of known passivating agents include the compounds of antimony, bismuth, tellurium, and thallium. However, to our knowledge, no one has suggested that zinc and its compounds, which are nontoxic and relatively inexpensive, are effective passivating agents. Although zinc has not been taught as a passivating agent, its use has been mentioned for other purposes.
One example is the teaching in "Catalysis" edited by P. H. Emmett (Reinhold Publishing Corp. 1954) at page 31 that a nickel hydrogenation-dehydrogenation catalyst can be poisoned by compounds of sulfur, selenium, tellurium, phosphorus, arsenic, antimony, bismuth, and zinc, and also by halides, carbon monoxide, mercury, lead, ammonia, pyridine, 1-ethyl-cyclopentane, oxygen, acetylene, hydrogen sulfide, phosphine, iron oxide, and silver dust.
Zinc has also been mentioned for uses in connection with the catalytic cracking of hydrocarbons. As discussed in detail below, zinc has been mentioned both as an oxidation catalyst and as a sulfur dioxide absorbent. In other words, zinc has been taught to have utility when present in the regeneration zone of a catalytic cracking process, but not when present in the reaction zone.